The present invention relates to a jack safety device.
A previous device provides one set of elastic stop-rotation keys engageable with a coupling groove provided above a hydraulically driven central stem shaft. Such a unit serves to prevent the operator from being injured by a heavy load that would drop down due to sudden retraction of the hydraulic stem shaft in case of failure or malfunctioning of the jack. In addition, the previous art provides one set of braking wires comprising braking wire, a pressing spring, a hollow-set key lid, an axis of the braking wire, plus two screw nails in addition to a spanner provided on a propelling stem, in order that it may rise up by a backpull of a stop-rotation key in case of a depressurization taking place, such that the hydraulically driven stem shaft may disengage and move back into the hydraulic tank. In addition, the previous art further incorporates a push block whose top accommodates a projecting rod attached by a retraction spring interconnecting another projecting rod attached to the tail end of the hydraulic tank so that the hydraulically driven stem shaft will be bound to be pulled back by a spring when it moves to retreat into the hydraulic tank.
The arrangement to provide for a correlated engagement between the stop-rotation key and coupling groove on the hydraulic stem shaft, as taught in the previous art, has surely the advantage of preventing sudden, unexpected falling of a load borne by the jack due to sudden or unexpected failure, and consequently retraction, of the stem shaft, thereby achieving better protection of the operator at work. Yet it has to be pointed out, as it has been noted by experience, that the axis of the braking wire employed in the previous art will and can wear out to rupture after prolonged service by pulling or dragging, just like what would happen to the braking wire of a bicycle or like vehicles upon prolonged and protracted service, that which would in all likelihood result in rupture and consequently failure of braking action. The same situation applies easily to the previous art of the device; in addition, as a spanner was fitted onto the propelling stem according to the previous art, the propelling stem was in turn locked to the machine bed at a slope of 60 degrees, which as such occupies some space and therefore will cause inconvenience in the shipping or handling practice.